Bioorthogonal transformations include four broad reaction types: 1,3-dipolar cycloadditions, Diels-Alder reactions, metal-catalyzed couplings, and nucleophilic additions. Outside of this space lies the [2+2+2] cycloaddition reaction, a choice for the rapid assembly of functionalized ring systems. In practice, such reactions are typically metal catalyzed as a means to overcome an otherwise significant entropic barrier. However, the highly strained hydrocarbon quadricyclane directly undergoes [2+2+2] cycloaddition with specific types of pi systems.
Literature
U.S. Pat. No. 7,808,619; U.S. Patent Publication No. 2009/0068738; U.S. Pat. No. 6,570,040